Dynamic strain aging (rate of work hardening) of steel wire occurs when the impurity atoms and dislocations interact during wire deformation. The dynamic strain aging for a coil spring of steel wire includes two aspects. One aspect is that the coil spring is dynamically strained and the other aspect is that the coil spring is aged. Depending on the combination of operating conditions, dynamic strain aging can occur during the coiling process. The specified temperature range in which the dynamic strain aging occurs depends on the strain rate, i.e. corresponding to the speed that the wire is pulled into the coil spring machine where the coil is formed. Increasing the strain rate, i.e. by winding the steel wire into a coil at a faster rate, typically raises both the lower and upper temperature limits associated with the dynamic strain-aging phenomenon. For example, at a strain rate of about 560 meters/minute (m/min), dynamic strain aging in the coil occurs by heating the coil to a stress relieving temperature in the range from between about 450° Centigrade (C.) to about 700° C. However, at a strain-rate of about 10 m/min to about 50 m/min, dynamic strain aging in the coil occurs by heating the coil to a much lower temperature range from between about 260° C. to about 300° C.
In metal wire containing interstitial solutes, such as carbon, nitrogen and oxygen in iron, the work hardening rate can become abnormally high during the coil formation. The work hardening rate of the metal wire can also be strain-rate and temperature dependent. Literature has documented that there is a maximum work hardening rate for the metal wire corresponding to a specific strain rate, above and below which the work hardening rate is lower. Further, if the temperature of the metal wire being coiled is raised, the strain rate at which maximum work hardening occurs also rises.
In steel, the dynamic strain-aging phenomenon is frequently called blue brittleness. It occurs approximately at the center of the temperature range in which the dynamic strain aging (heat and cracks) occurs, i.e. where the elongation of the steel wire becomes very small or the wire is drawn with a minimum amount of elongation. In dynamic strain aging, the necking, i.e. reduction in size of the steel wire cross-section, starts under tensile loading conditions at a relatively small strain and increases to highly concentrated strain conditions. The steel wire, however, does not become brittle and the reduction of area does not reach a minimum. Such a minimum in the elongation is not observed in the pure metals or alloys without any interstitial atoms.
There is a need in forming coil springs of steel to reduce the breaking of the wire during manufacture or service due to brittleness caused by the manufacture process.